1. Field of the Invention
The present invention relates to an optical semiconductor integrated circuit device incorporating a photodiode for converting a light signal to an electric signal.
2. Description of Related Art
An optical semiconductor device in which a light receiving photodiode is integrated together with its peripheral circuit has been used on a receiving side of light signal transmitting device for transmitting a light signal in the form of an infrared ray as well as in a light signal reading device of an optical pickup apparatus. The integrated circuit device can be expected to realize a cost-down compared to a circuit device fabricated by hybridization discrete parts thereon. The integrated circuit device has a merit that it shows a strong resistance to noises due to external electromagnetic field. In the semiconductor device incorporating the photodiode, regions other than a region where the photodiode is arranged needs to be shadowed from incidence light so that excess photocurrent by the incidence light is not produced to the peripheral circuit.
FIG. 9 shows an example of the semiconductor device incorporating the photodiode (see Japanese Patent Laid-Open No. Hei 10 (1998)-107242).
In FIG. 9, a photodiode 51 and an NPN transistor 52 are shown. The semiconductor device includes a P-type single crystal silicon semiconductor substrate 53, a non-doped first epitaxial layer 54 formed on the substrate 53 by a chemical vapor deposition method, and an N−-type second epitaxial layer 55 formed on the epitaxial layer 54 by the chemical vapor deposition method. A resistivity of the substrate 53 is 2 to 4 Ω·cm. A thickness of the first epitaxial layer 54 is 5 to 10 μm, and a resistivity thereof is 50 Ω·cm or more. A thickness of the second epitaxial layer 55 is 2 to 5 μm, and a resistivity thereof is about 1 Ω·cm.
The substrate 53, the first epitaxial layer 54 and the second epitaxial layer 55 are isolatedly partitioned into a first island region 57 for forming a photodiode 51 and a second island region 58 for forming an NPN transistor 52 by a P-type isolation region 56 reaching from a surface of the epitaxial layer 55 to the substrate 53. The isolation region 56 is composed of a first isolation region 59 diffusing from a surface of the substrate 53 upward and downward, a second isolation region 60 diffusing from a surface of the first epitaxial layer 54 upward and downward, and a third isolation region 61 diffusing from a surface of the second epitaxial layer 55. Each of the first and second island regions 57 and 58 are fully surrounded by a junction boundary between the isolation region 56 and each of the first and second epitaxial layers 54 and 55, and a junction boundary between the substrate 53 and the first epitaxial layer 54.
In the first island region 57, an N+-type diffusion region 62 of the photodiode 51 is formed. The substrate 53 constitutes a positive portion of a PIN (Positive-Intrinsic-Negative) junction. The first and second epitaxial layers 54 and 55 constitute an intrinsic portion of the PIN junction. The N+-type diffusion region 62 is a negative portion of the PIN junction. With this structure, the PIN junction is formed, and the photodiode 51 is formed. The NPN transistor 52 is formed in the second island region 58. The NPN transistor 52 is constituted by an N-type collector region 66, an N-type buried layer 63, a P-type base region 64 and an N-type emitter region 65.
The N-type collector region 66 is formed so as to be connected to the N-type buried layer 63 from a surface of the second epitaxial layer 55. The N-type buried layer 63 is formed so as to straddle a boundary between the first and second epitaxial layers 54 and 55. The P-type base region 64 is formed in a surface of the second epitaxial layer 55. The N-type emitter region 65 is formed in a surface of the base region 64.
The surface of the second epitaxial layer 55 is covered with an oxide film 67, and contact holes are formed by partially perforating the second epitaxial layer 55. The contact holes are formed respectively on the emitter region 65 of the NPN transistor 52, the P-type base region 64 thereof, the collector region 66 thereof, the N+-type diffusion region 62 of the photodiode 51, and the isolation region 56 thereof. A collector electrode 48, a base electrode 49, and an emitter electrode 50 are provided in a region of the NPN transistor 52 through the contact holes. In the N+-type diffusion region 62 of the photodiode 51, a cathode electrode 46 is provided, and in the isolation region 56, an anode electrode 47 is provided.
An, oxide film 68 is formed on the oxide film 67 and the electrodes 46, 47, 48, 49 and 50. On the oxide film 68, an Al layer 45 is formed as a light shading film. The Al layer 45 opens in a portion of the photodiode 51. A thickness of the oxide film on the photodiode 51 is approximately equal to that on the NPN transistor 52. This technology is described for instance in Japanese Patent Laid-Open No. Hei 10 (1998)-107242.
As a recording density is more increased, a wavelength used becomes shorter, and blue laser having a wavelength of 405 nm has been recently focused on.
However, in a photodiode for the blue laser, resin used for a transparent package sealing a chip absorbs energy of incidence light, and the package is burnt. Accordingly, a hollow package for airproofing the chip without using resin needs to be adopted as an IC package. In such a structure, an insulating film on a light receiving region of the photodiode is exposed to air in the hollow package. Then, a reflection of the incidence light occurs in a surface of the insulating film due to difference of a refraction factor between air and the insulating film, and the reflection of the incidence light depends on a thickness of the insulating film. As a result, a problem that a sensitivity of the photodiode is influenced by variations of the thickness of the insulating film has been known. In order to solve this problem, the insulating film on the light receiving region should be removed. On the other hand, it is preferable that the foregoing light shadowing film should cover a portion in the vicinity of the photodiode to prevent entering of unnecessary light. However, a sum of thicknesses of insulating films more increases due to a high integration and a multilayered structure of recent LSIs. When an opening is provided in such an insulating film and a light shadowing film is formed on the insulating film, there is a problem that the light shadowing film is broken by a step of the opening portion.